This invention relates generally to improved methods and apparatus for investigating subsurface earth formations traversed by a borehole, and more particularly to an improved technique for processing formation measurements to obtain displacements between measurements for use in correlating logs.
It is often desirable to correlate or compare curve shapes of two or more well logging curves which have been generated either during the same or different traversals of the logging instrument through the same borehole or, alternatively, during traversals through adjacent boreholes to locate corresponding data points on the curves.
One example of the reason for this may be to check or compare a re-logging of a well against a prior log of the same or different parameters to insure that all measurements are on depth. Another example might be in field studies where well-to-well depth correlations are desired. Yet another example might be in the case of formation dip measurements wherein a plurality of measurements are made during one borehole pass.
In the latter case, a logging instrument is provided having four movable arms spaced ninety degrees apart in azimuth, each having a pad in contact with the borehole wall which carries an electrode system for making a shallow focused formation resistivity measurement.
Normally, the shapes of each logging curve measurement thus generated by the pads are similar since they are measuring characteristics of portions of the formation relatively adjacent one another.
However, due to such things as the logging sonde not always being oriented perpendicular to formation beds (for example because the beds are inclined relative to the sonde), although the shapes of each measurement may appear similar, they may appear offset in depth. This is because one of the measuring pads will reach the bed and thus generate a characteristic signature prior to another pad.
As is well known in the art, the amount of such offsets in the pad signals relative to one another provides valuable information about the amount and direction of formation dip. Thus, once again it is necessary to find ways to compare or correlate the logging curve shapes to determine the offsets.
Several methods have been attempted over the years to correlate two or more logging curves. The oldest method was simply optical correlation by an experienced individual whereby visual comparisons were simply made between portions of the logs. While this method was often very reliable, it was obviously extremely time consuming, particularly if high resolution was desired or large depth intervals were involved, and moreover, the method further depended upon the subjective human abilities of the particular analyst.
Another group of methods known as "fixed interval correlation" utilized a statistically defined cross-correlation coefficient in comparing successive intervals of finite length on two measurement curves as a measure of curve similarity.
Fixed interval correlation has also evidenced several difficiencies in log analysis applications including insufficient depth resolution and computational inefficiency inasmuch as a large number of computations was required. The method was also particularly unsuited to formation dip measurements exhibiting unreliability in complicated stratification, for example, and loss of dip variations when smaller than the correlating interval.
Moreover, this correlation interval was preset and thus not adapted continuously to the current geological context, causing missed correlations and limitations on resolutions. Still further, because the dip calculations were attributed to an interval and not to the bedding, only bulk directional properties over each particular interval were described rather than the basic cause of correlation, e.g., bedding. Thus, true correlations were often missed or high cross-correlation coeffients were noted for curve features which did not correlate.
Still another log correlation method commonly referred to as "point-to-point" correlation has been attempted. In this method, pattern recognition or classificiation is employed whereby pattern vectors of each curve to be correlated are analyzed against a "catalog" of standard patterns.
However, although improvement in depth resolution may have been noted, this method too has been found deficient particularly with formation dip applications in its noise susceptibility, e.g., failure to distinguish between regular and random features in data which result in excessive scatter and gaps in formation dip determinations.
Accordingly, the present invention overcomes these and other deficiencies of the prior art by providing an improved method and apparatus for utilizing statistical and frequency distribution of the measurement data to determine well log correlations.